The present invention relates to a high strength pipe of ethylene polymer having a bimodal molecular weight distribution.
Polyethylene is used in large amounts for the production of pipes, for example for gas and water transport systems, because a material having high mechanical strength, high corrosion resistance and good long-term stability is required for such pipes. Many publications describe materials having a very wide range of properties and processes for their preparation.
EP-A-603,935 has already described a molding material which is based on polyethylene, has a bimodal molecular weight distribution and is suitable, inter alia, also for the production of pipes. However, pipes produced from the molding materials according to this publication are still unsatisfactory with regard to their continuous load capacity under internal pressure, their stress cracking resistance, their low-temperature notched impact strength and their resistance to rapid crack growth.
In order to obtain pipes having balanced mechanical properties and hence an optimum combination of properties, it is necessary to use a raw material which has an even broader molecular weight distribution. Such a raw material is described in U.S. Pat. No. 5,338,589 and is prepared using a highly active catalyst which is disclosed in WO 91/18934 and in which the magnesium alcoholate is used as a gel-like suspension. It has surprisingly been found that the use of this material in shaped articles, in particular in pipes, permits a simultaneous improvement in the properties of rigidity and tendency to creep on the one hand and stress cracking resistance and toughness on the other hand, which are usually opposed in semicrystalline thermoplastics.